Powder coating technology has generally evolved over several years into various types of coating techniques. Certain advanced techniques involve the use of electrostatic technology to adhere a powder, such as a resinous polymer or paint, to a desired workpiece preferably with a uniform thickness. The initial adherence of the powder to the surface of the workpiece takes place due to the attraction created by different electric charges existing on the workpiece and the particles of powder. After the initial electrostatic powder coating is formed, the coating is cured using techniques such as heat, infrared light or ultraviolet light, to fully adhere the coating to the workpiece.
Conventional electrostatic techniques for initially adhering the powder particles to a workpiece include two general types. The first type involves electrostatically charging the powder particles emanating in a cloud from a fluidized powder bed. When an electrically conductive, grounded workpiece is placed within the emanating cloud of electrostatically charged particles, the charged particles become attracted to the outer surfaces of the workpiece and form the initial layer of coating. The workpiece may be manually placed within a powder coating hood containing the electrostatically charged powder cloud or may be on a conveyor system or otherwise moved continuously through the hood or other powder coating area. The second powder coating technique utilizes a spray gun discharging electrostatically charged powder particles. An electrode at the gun nozzle may electrostatically charge the particles or the gun may emit a stream of powder particles charged by air upstream of the gun nozzle. With electrostatic guns, the workpieces are again grounded and typically placed within a spray coating hood during a coating operation to contain and collect excess sprayed powder.
A significant, continuing problem associated with electrostatically coating workpieces concerns achieving a uniform coating on the workpiece. With a workpiece oriented or moving horizontally, for example, this problem particularly exists with respect to top to bottom uniformity. That is, the lower surfaces of the workpiece tend to develop a heavier coating build than the upper surfaces. It is believed that this effect may be attributed to stratification, or a progressive decrease in density of particle distribution upwardly over the bed. Also, the charge on the particles may reduce in strength with increased remoteness from the voltage source and/or due to dissipation of the initial charge.
Various solutions have had different amounts of success in dealing with these problems. Some apparatus deal with these problems by creating a swirling or vortex-like flow of air and powder about the workpiece to more evenly distribute the powder particles on all outer surfaces of the workpiece. For example, U.S. Pat. Nos. 4,606,928; 4,808,432; and 5,773,097, each assigned to the assignee of the present invention, are all concerned with apparatus that distribute charged powder particles more uniformly about a workpiece, such as a continuous elongate strand of wire, cable, tubing or other like material. While devices such as these have met with significant success, it would still be desirable to provide improvements relative to achieving cost efficiencies, size reduction and reduced overall complexity of the devices as well as continued improvement in coating uniformity.